Microelectromechanical systems (“MEMS”) are used in a growing number of applications. For example, MEMS currently are implemented as gyroscopes to detect pitch angles of airplanes, and as accelerometers to selectively deploy air bags in automobiles. In simplified terms, such MEMS devices typically have a structure suspended above a substrate, and associated electronics that both senses movement of the suspended structure and delivers the sensed movement data to one or more external devices (e.g., an external computer). The external device processes the sensed data to calculate the property being measured (e.g., pitch angle or acceleration).
Many types of MEMS sensors, such as those discussed above, are manufactured by means of conventional surface micromachining (“SMM”) techniques. As known by those skilled in the art, surface micromachining techniques build material layers on top of a substrate using additive and subtractive processes. Unlike processes that use silicon-on-insulator (“SOI”) wafers, these processes do not require wafer to wafer bonding.
U.S. Pat. No. 5,326,726 (“Tsang”) and U.S. Pat. No. 5,828,115 (“Core”) discuss two conventional SMM processes. The sensors shown by both patents have a conductive layer that connects structure to circuitry. For example, FIGS. 1 and 2 of the Core patent show a sensor having a ground plane 50 that electrically connects a mass 12 to a contact 68 via a diffused runner 66 embedded in its substrate 36. As noted at lines 62-64 of column 2 of the patent, this arrangement creates a “large” parasitic capacitance between the ground plane 50 and the substrate 36. In fact, this parasitic capacitance can corrupt electronic signals transmitted through the ground plane 50.
A semiconductor junction within the Core sensor significantly contributes to this parasitic capacitance. More specifically, the point where the diffused runner 66 meets the substrate 36 appears to create a semiconductor junction similar to that in a diode. Under certain conditions, that junction undesirably can begin conducting. For example, if the sensor is near a heat source (e.g., under the hood of an automobile), the increased temperatures can cause the junction to leak, thus causing unintended conduction. As a further example, relatively high voltages applied to the sensor also can cause the junction to begin conducting.